This invention relates to gas dynamic lasers and relates to overall devices for returning gas flow in a closed loop. While a fraction of the flow is discharged overboard, a majority of the flow is recirculated. Application Ser. No. 772,072, filed Oct. 29, 1968 for a CLOSED CYCLE DEVICE and Application Ser. No. 336,000, filed Feb. 26, 1973 for a CLOSED CYCLE GAS DYNAMIC LASER contain background material.